1. Field of the Invention
The present invention generally relates to a method and apparatus for rendering harmless a detonation front traveling in a pipeline and, more particularly, to a detonation safety device having a flame-arresting device that hinders the transmission of the detonation flame.
2. Background Description
The propagation of an explosion in a flammable gas mixture in a container and/or pipeline system can occur as detonation or as deflagration. In the case of detonation, the flame front and the shock front created by the pressure wave of the explosion are superimposed such that flame propagation velocities may reach several thousand meters/second (m/s) and combustion pressures in the shock direction may reach upwards of 100 bars. In the case of deflagration, the shock waves precede the flame front such that flame propagation velocities of deflagration are in the order of several hundred m/s and the combustion pressures in the shock direction are upwards of 10 bars (with an original pressure of the mixture of one bar).
Several methods are known to avoid the destructive effects of detonations. These methods attempt to weaken and/or end the detonation, preferably by transforming the detonation into a deflagration prior to arrival at a flame-arresting device. In many instances, the flame-arresting device is combined with xe2x80x9cdetonation brakesxe2x80x9d and/or xe2x80x9cdetonation shock catchersxe2x80x9d (e.g., detonation absorbers), and further includes a number of long and narrow slots. These long and narrow slots attempt to cool the flame until the flame becomes extinct.
By way of example, DE-PS 1192 980 (German patent) describes a detonation safety device consisting of a detonation brake and a flame arresting device. In this device, a detonation front propagating in a pipeline is split by a convex outer surface of a cylindrical wall and reaches an expansion space that has a volume comparatively greater than the pipeline. A second semicircular cylindrical wall of a smaller diameter than the cylindrical wall is provided such that two facing free wall parts of the cylindrical wall and the second semicircular cylindrical wall are overlapped, thereby forming a labyrinth of various turns. In order to extinguish the detonation, the split detonation front travels through the labyrinth until it reaches the flame-arresting device. The flame-arresting device is placed in an exit housing and angled at 90 degrees to the pipeline in which the detonation initially propagated.
However, in these known devices as described with reference to DE-PS 1192 980, the split detonation fronts create a secondary detonation, especially under unfavorable mixture composition conditions. It is thus necessary to size the flame-arresting device in such a way that it performs the flame extinguishing function even in the secondary detonation case. In order to accomplish the flame extinguishing function, the flame extinguishing slots of the arresting device must be adequately long and narrow so as to realize a relatively high pressure loss during normal pipeline operation. However, these long and narrow slots require high maintenance which can be costly and time consuming.
By way of further example, DE 195 36 292 C2 teaches a split of the detonation front into a main front and a secondary front. In the case of DE 195 36 292 C2, the main front is conducted into the expansion space with a longer transit time so that upon entry into the expansion space the main front contains combustion gases of the secondary front. It is noted, however, that the splitting of the detonation into main and secondary fronts such that the main front needs a longer transit time to reach the expansion space also requires many turns which thus results in a minimum volume for the detonation safety device. Thus, there is a need to use a pre-installed shock buffer for at least the main front. It is noted, however, that the use of the shock buffer results in a relatively high fabrication cost especially when the detonation safety device is impacted by detonation fronts from both sides in which case the shock buffers must be installed on both sides of the flame-arresting device.
In principle, it would be possible to use a flame-arresting device without a shock buffer. However, the slots of the arresting device must be quite long and narrow in order to achieve adequate safety, resulting in high pressure losses across the arresting device. If flame arresting devices with low pressure drop are used, the flame front entering the arresting device can push lighter non-combusted mixtures through the arresting device which would thus result in higher stream velocities and the creation of turbulence in the flame extinguishing slots in the flame front travel direction. The higher stream velocities would thus increase the combustion velocity and reduce the extinction capability and the flame arresting safety of the device. It is further known that if the arresting devices with high damming capability created by long and narrow slots are used to provide high flame arresting safety, significant operational drawbacks of high pressure losses will result in the arresting devices.
It is therefore an object of the present invention to provide a detonation safety device which is built with simple and inexpensive components.
It is a further object of the present invention to provide a detonation safety device which does not have large pressure drops.
It is still a further object of the present invention to provide a detonation safety device which provides a high degree of flame arresting safety.
It is another object of the present invention to provide a detonation safety device which does not use a shock buffer.
According to the invention, there is provided a detonation safety device which renders harmless a detonation front traveling in a pipeline by the use of a flame-arresting device. The detonation safety device includes a housing built into a pipeline and/or container system having a specific diameter. The flame-arresting device is housed in the housing and hinders the detonation of the flame of the detonation front.
The diameter of the flame-arresting device is significantly larger than the pipe diameter thus providing a desired low pressure drop. Since the diameter of the flame-arresting device is larger that the pipeline diameter, it is appropriate to convey the detonation front as several part-fronts to various parts of the flame arresting device. This arrangement also allows an even distribution of flowing gases over the comparatively large surface area of the arresting device during normal operations.
A pipe stub extends from the pipe and into the housing. The outlet of the pipe stub is proximate to the flame-arresting device and creates an open space so that a detonation front traveling through the pipe stub impinges only on a portion of the flame-arresting device. The pipe stub is placed near the flame-arresting device such that the portion being impinged by the front is essentially equal to the pipeline diameter. The flame-extinguishing operating mode becomes more effective as the end of the pipe stub is placed closer to the flame-arresting device.
The pipe stub may also include small (relative to the pipe diameter) connection openings between the pipe stub and the surrounding open space which causes pre-combustion of the detonation front in the expansion space. The pre-combusted gases avoid the tendency of a renewed detonation front in the expansion space, especially one caused by a reflection off the end wall of the expansion space furthest away from the arresting device. In this way, the length of the expansion space can be reduced. In still further embodiments of the present invention, several pipe stubs are provided, where each of the pipe stubs have a smaller diameter than the diameter of the pipeline.
Preferably, the flame-arresting device has a total diameter of at least double the front impingement diameter so that low pressure drops are achieved during normal operations. A lower limit for the reduction of the gap between the end of the pipe stub and the flame-arresting device results from the need that, during normal operations, the total cross-section of the flame-arresting device be uniformly impinged at the usual, normally relatively low, flow velocities. Within the limits of these boundary conditions, the distance between the outlet of the pipe stub and the flame-arresting device is equal to or larger than one third of the pipe diameter and equal to or smaller than the pipe diameter.
The detonation front is expanded before reaching the flame-arresting device in such a manner that a deflagration ensues and impinges on the outer cross-section of the flame-arresting device. In the embodiments of the present invention, a small portion of the detonation front is diverted to an expansion space for pre-burning so that pre-combusted gases can prevent a renewed formation of a detonation front in the expansion space.
By using the configuration of the present invention, the detonation safety device of the present invention may be used without a shock buffer.